The invention concerns a photodiode array having a plurality of channel. Such a photodiode array is used, for example, for measuring the absorption spectrum of a sample substance to derive information concerning the chemical composition of the sample and the quantities of individual constituents in the sample.
A photodiode array of this kind is known in the art through European patent EP 0 519 105 B1. This conventional photodiode array can be used in a liquid chromatograph for analyzing the substances eluting from the chromatographic column. It comprises a light source emitting a broad spectrum of ultraviolet and visible radiation and an optical system for focussing the beam onto a sample cell through which the sample substances to be analyzed flow. Depending on the specific substances flowing through the cell, the sample absorbs certain characteristic spectral portions of the radiation entering the sample cell so that the spectral composition of the radiation leaving the cell is indicative of the sample substances.
In such a spectrometer, the spectrum of the radiation leaving the sample cell is extracted using a diffraction grating disposed in the optical path behind the cell. The diffraction grating directs light rays of differing wavelengths into different directions. A linear photodiode array is disposed to receive the light diffracted by the grating. Each diode thereby receives light corresponding to a different wavelength range. The electrical signals produced in each photodiode by the impinging light are read out by a read-out circuit and converted to digital data values representative of the intensity of the light incident on the respective diode. These data values are then displayed as a function of wavelength in any convenient form, for example on a CRT screen.
The photodiode array is a semiconductor device and comprises a plurality of photosensitive elements connected via electronic switches to a common output line, e.g. a video line, which in turn is connected to a charge amplifier. Each photosensitive element has an associated capacitor representing the junction capacitance of the photodiodes. The combination of a photosensitive element and associated capacitor is also referred to as a "photocell".
Light impinging on the photosensitive material generates charge carriers discharging these capacitors. The capacitors of the photocells are initially charged to a predetermined value and are discharged by the photocurrent generated by the photocells when light impinges thereon. The amount of charge needed to recharge the capacitors to their original values causes a voltage change at the output of the charge amplifier, a signal indicating the light intensity on the photodiode.
A photodiode array comprises a plurality of photocells, each generating these output signals, which are processed sequentially. The photodiode array usually operates in an integrating mode (self-scanning and random access photodiode arrays). The distribution of the output signals over time is associated with the problem of spectral distortion. In particular, for spectrophotometers used to detect sample substances eluting from the column of a liquid chromatograph, the sample to be analyzed changes as a function of time. Since the signals from the individual photocells are processed sequentially, the output signals caused by light beams of different wavelengths simultaneously impinging on the photodiodes are therefore evaluated in a time distributed fashion.
Another problem is that a single A/D converter is normally used to sequentially convert the signals from individual photodiodes of the photodiode array. Since the number of photodiodes is usually very large, i.e. 1024 photodiodes, the conversion rate of the A/D converter has to be very high, e.g. above 100 kHz, to ensure high measuring accuracy. Such A/D-converters are rather complex and expensive.
A parallel photodiode array architecture is therefore preferred in accordance with EP 0 519 105. The signals from each channel, having its own converter, are simultaneously generated. Simpler A/D converters can be used for each channel and the measuring accuracy of time variable sample concentrations is improved.
The use of a charge balance type of photodiode array is preferred to improve integration of the photodiode array, e.g. onto one single silicon chip. This type of photodiode array uses an integrator circuit to accumulate the charge delivered by the photocurrent and removes the charge accumulated within a predetermined time interval in defined charge packets using a switchable dumping capacitor. The frequency of charge dumps required to keep the system in balance is proportional to the photocurrent generated by the individual photodiode. Each photodiode is connected to the summing node of an integrator, which continuously accumulates the charge corresponding to the photocurrent for effecting the A/D conversion. The output signal of the integrator is periodically compared to a predetermined signal level, i.e. by a suitable comparator and, in response to these comparisons, charge dumps to and/or from the integrator are executed to keep the output signal near a predetermined level. The number of such dumps is counted, i.e. by a logical counter during a predetermined time interval. The number determined is a digital signal representing the actual photocurrent.
This integrator circuit usually comprises an operational amplifier. This causes problems relating to the input offset voltage and the input offset drift of this operational amplifier. The input offset voltage results from the circuit design, random offset, and from mismatches on the silicon chip. Additional problems are caused by noise e.g. so-called flicker noise and thermal noise, associated with the input stage of this operational amplifier. A special problem concerning the noise performance is related to the fact that the noise behavior is not in equipartition over the whole frequency-spectrum. The flicker noise contribution changes as 1/f-. Therefore the noise contribution is particularly high at lower signal frequencies.
In a preferred embodiment, the charge balance type of photodiode array comprises C-MOS-technology to reduce the voltage gain of the operational amplifier and increase the influence of an input offset voltage.
The flicker noise performance of a MOSFET-circuitry could be improved by increasing the size of the gate area of the MOSFET's. The thermal noise performance mainly depends on the conductance of the input-MOSFET's of the circuitry which, in turn, depends on the ratio of channel width to channel length. In consequence thereof, an enlargement of the gate area is desirable to improve the noise performance, but is associated with increased space and therefore power demand. This is in conflict with attempts to always develop more highly integrated chips.